The excited state dynamics of correlated electron and electron-phonon systems triggered by an oscillating electric-field pulse of large amplitude are theoretically investigated. A negative-temperature state and inversion of electron-electron and elec
tron-phonon interactions are induced even by a symmetric monocycle pulse. This fact is numerically demonstrated, using the exact diagonalization method, in a band-insulator phase of one-dimensional three-quarter-filled strongly dimerized extended Peierls-Hubbard and Holstein models. When the total-energy increment is maximized as a function of the electric field amplitude, the occupancy of the bonding and antibonding orbitals is inverted to produce a negative-temperature state. Around this state, the dependences of time-averaged electron-electron and electron-phonon correlation functions on interaction parameters are opposite to those in the ground state.
A negative differential resistance (NDR) in a one-dimensional band insulator attached to electrodes is investigated. We systematically examine the effects of an electrode bandwidth and a potential distribution inside the insulator on current-voltage
characteristics. We show that, in uncorrelated systems, the NDR is generally caused by a linear potential gradient as well as by a finite electrode bandwidth. In particular, the former reduces the effective bandwidth of the insulator for elastic tunneling by tilting its energy band, so that it brings about the NDR even in the limit of large electrode bandwidth.
We theoretically investigate charge order and nonlinear conduction in a quasi-two-dimensional organic conductor beta-(meso-DMBEDT-TTF)2PF6 [DMBEDT-TTF=dimethylbis(ethylenedithio)tetrathiafulvalene]. Within the Hartree-Fock approximation, we study eff
ects of structural distortion on the experimentally observed checkerboard charge order and its bias-induced melting by using an extended Hubbard model with Peierls- and Holstein-types of electron-lattice interactions. The structural distortion is important in realizing the charge order. The current-voltage characteristics obtained by a nonequilibrium Greens function method indicate that a charge-ordered insulating state changes into a conductive state. Although the charge order and lattice distortions are largely suppressed at a threshold voltage, they remain finite even in the conductive state. We discuss the relevance of the results to experimental observations, especially to a possible bias-induced metastable state.
We theoretically discuss the mechanism for the peculiar nonlinear conduction in quasi-two-dimensional organic conductors theta-(BEDT-TTF)2X [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] through the melting of stripe-type charge order. An extended
Peierls-Hubbard model attached to metallic electrodes is investigated by a nonequilibrium Greens function technique. A novel current-voltage characteristic appears in a coexistent state of stripe-type and nonstripe 3-fold charge orders, where the applied bias melts mainly the stripe-type charge order through the reduction of lattice distortion, whereas the 3-fold charge order survives. These contrastive responses of the two different charge orders are consistent with the experimental observations.
Nonequilibrium states induced by an applied bias voltage (V) and the corresponding current-voltage characteristics of one-dimensional models describing band and Mott insulators are investigated theoretically by using nonequilibrium Greens functions.
We attach the models to metallic electrodes whose effects are incorporated into the self-energy. Modulation of the electron density and the scalar potential coming from the additional long-range interaction are calculated self-consistently within the Hartree approximation. For both models of band and Mott insulators with length L_C, the bias voltage induces a breakdown of the insulating state, whose threshold shows a crossover depending on L_C. It is determined basically by the bias $V_{th}sim Delta$ for L_C smaller than the correlation length $xi=W/Delta$ where W denotes the bandwidth and $Delta$ the energy gap. For systems with $L_Cgg xi$, the threshold is governed by the electric field, $V_{th}/L_C$, which is consistent with a Landau-Zener-type breakdown, $V_{th}/L_Cpropto Delta^2/W$. We demonstrate that the spatial dependence of the scalar potential is crucially important for this crossover by showing the case without the scalar potential, where the breakdown occurs at $V_{th}sim Delta$ regardless of the length L_C.
Photoinduced melting of horizontal-stripe charge orders in quasi-two-dimensional organic conductors theta-(BEDT-TTF)2RbZn(SCN)4[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] and alpha-(BEDT-TTF)2I3 is investigated theoretically. By numerically so
lving the time-dependent Schrodinger equation, we study the photoinduced dynamics in extended Peierls-Hubbard models on anisotropic triangular lattices within the Hartree-Fock approximation. The melting of the charge order needs more energy for theta-(BEDT-TTF)2RbZn(SCN)4 than for alpha-(BEDT-TTF)2I3, which is a consequence of the larger stabilization energy in theta-(BEDT-TTF)2RbZn(SCN)4. After local photoexcitation in the charge ordered states, the growth of a photoinduced domain shows anisotropy. In theta-(BEDT-TTF)2RbZn(SCN)4, the domain hardly expands to the direction perpendicular to the horizontal-stripes. This is because all the molecules on the hole-rich stripe are rotated in one direction and those on the hole-poor stripe in the other direction. They modulate horizontally connected transfer integrals homogeneously, stabilizing the charge order stripe by stripe. In alpha-(BEDT-TTF)2I3, lattice distortions locally stabilize the charge order so that it is easily weakened by local photoexcitation. The photoinduced domain indeed expands in the plane. These results are consistent with recent observation by femtosecond reflection spectroscopy.
We discuss photogenerated midgap states of a one-dimensional (1D) dimerized Mott insulator, potassium-tetracyanoquinodimethane (K-TCNQ). Two types of phonon modes are taken into account: intermolecular and intramolecular vibrations. We treat these ph
onon modes adiabatically and analyze a theoretical model by using the density-matrix renormalization group (DMRG). Our numerical results demonstrate that the intermolecular lattice distortion is necessary to reproduce the photoinduced midgap absorption in K-TCNQ. We find two types of midgap states. One is a usual polaronic state characterized by a localized elementary excitation. The other is superposition of two types of excitations, a doped-carrier state and a triplet-dimer state, which can be generally observed in 1D dimerized Mott insulators, not limited to K-TCNQ.
